Because of their good heat resistance, corrosion resistance, electrical conductivity and mechanical strength, carbonaceous materials are in broad use as fuel cell electrode materials, electromagnetic shielding materials and so on. Among carbonaceous materials, materials in the film or sheet form, such as graphite sheets, can be bonded to, for example, the wall surface to impart heat resistance, corrosion resistance and electromagnetic shielding properties.
However, since a graphite sheet is generally produced by a method which comprises subjecting naturally-occurring graphite serially to acid treatment and heat treatment, mixing the thus-treated graphite with a binder, and compression-molding the mixture into a film or sheet, it is not only non-porous but also is fairly low in electrical conductivity, heat conductivity, mechanical strength and cushioning characteristics, thus being seriously restricted in application.
Meanwhile, a carbon plate can be produced by a process which comprises mixing a textile fiber which can be carbonized or graphitized and/or a carbon fiber with a particulate binder which can be carbonized or graphitized, forming the mixture into a plate under the application of heat and pressure and subjecting the plate to carbonization or graphitization. This kind of carbon plate can be used as, for example, a fuel cell electrode material.
Unlike other kinds of power generating devices, the fuel cell mentioned above is characterized in that the evolution of pollutants such as SOx, NOx and dust is minimal and, moreover, in that it is scarcely a source of noise. Among the known types of fuel cells, the phosphoric acid electrolyte fuel cell comprises a stack of separator-isolated unit cells each comprising a porous negative electrode and a porous positive electrode as disposed on respective sides of the electrolyte. For the provision of gas passageways, usually the surfaces of the negative and positive electrodes are formed with grooves by machining.
For an improved efficiency of conversion to electric energy, the negative and positive electrodes should provide for free control of pore distribution and be high in gas permeability. Further requirements are good electrical conductivity, heat conductivity, mechanical strength, and resistance to phosphoric acids at the operating temperature.
Japanese Patent Publication No. 36670/1989 (JP-B-1-36670) discloses a method for manufacturing a fuel cell electrode plate which comprises dry-mixing a binder, such as a phenolic resin, a carbon fiber and a particulate thermoplastic resin, molding the mixture into a sheet under pressure by means of a hot roll or a hot press and subjecting the sheet to carbonization or graphitization.
However, this method has the disadvantage that because the carbon fiber which is fibrous cannot be uniformly blended with the binder and the thermoplastic resin which are particulate, a segregation tends to occur among the carbon fiber and the binder and the thermoplastic resin in dry-mixing stage and the segregated binder and thermoplastic resin tend to agglomerate in the course of pressure-molding of the particulate composition to reduce the homogeneity of the molding. Furthermore, the segregated thermoplastic resin is re-softened in the carbonization or graphitization stage. The above segregation of the binder and the thermoplastic resin and softening of the thermoplastic resin in two episodes lower the homogeneity of the electrode material. Probably owing to this low homogeneity, the electrode material obtained is not only low in heat conductivity but shows local variations in flexural strength, compressive strength and gas permeability. Further, the segregation of the binder and the thermoplastic resin results in a non-uniformity of pore size distribution of the electrode material. Particularly in the manufacture of electrode plates of reduced thickness, it is difficult to obtain uniform pores.
Japanese Patent Application Laid-open No. 174359/1991 (JP-A-3-174359) discloses a method comprising mixing a carbon fiber with a particulate binder, processing the mixture into a paper-like web, pressure-molding the web and subjecting the molding to carbonization or graphitization. This method, however, has the disadvantage that the molding under heat and pressure must be carried out using a low pressure setting in order to secure a porosity of 60 to 80%. If the molding is carried out under such low pressure, the attainable interfilament bond strength is so low that the sintered electrode material shows only a low flexural strength, i.e. 1 kgf/mm.sup.2 at most, and low compressive strength, i.e. 0.4 kgf/mm.sup.2 at most, thus failing to meet the performance requirements of electrodes for phosphoric acid electrolyte fuel cells. Moreover, such electrode material has a high volume resistivity in thickness direction and a low heat conductivity.
Japanese Patent Application Laid-open No. 76821/1991 (JP-A-3-76821) discloses a method for manufacturing an electrode material which comprises mixing an organic precursor fiber convertible carbon fiber with a pulp, an organic polymer as a binder, etc., processing the mixture into a paper-like web, and sintering the web to provide an electrode material. However, the carbonization yield (residual carbon rate) of the organic fiber is as low as 10 to 30%. Therefore, the resulting electrode material shows a considerable shrinkage as compared with a corresponding molded material and assuming that an electrode plate measuring 1-3 mm in thickness and 1 m square is manufactured, it undergoes cracking, curling or twisting and, hence, no sufficient uniformity can be expected. Moreover, since the rate of shrinkage in thickness direction is high, local variations in gas permeability and volume resistivity are inevitable, thus failing to ensure a sufficient homogeneity.
In addition, machining is required for forming grooves on the surface of the electrode materials manufactured by the above-mentioned prior art methods. Therefore, the production process for electrodes is complicated. Moreover, since carbonaceous electrode plates are hard, groove-shaped gas passageways can hardly be formed with high efficiency and high accuracy and, moreover, the possible incidence of cracks in the electrode material due to machining detracts from the production efficiency of electrodes.
Meanwhile, in order to realize reductions in weight of carbonaceous materials, it is instrumental to increase their porosity but a reduction in weight is reflected in an amplified decrease in mechanical strength.
Referring to a porous composite sheet, Japanese Patent Publication No. 55618/1992 (JP-B-4-55618) discloses a low-density fiber-reinforced thermoplastic composite which can be used advantageously as a light-weight resin sheet with high flexural strength and flexural rigidity. This composite material can be obtained by subjecting a compressed fiber-reinforced composite containing a thermoplastic synthetic resin and a reinforcing fiber to heat treatment for expansion. Japanese Patent Publication No. 17249/1993 (JP-B-5-17249) discloses a method for producing a porous composite sheet of open-cell structure with high mechanical characteristics which comprises impregnating a reinforcing web, such as a nonwoven polyester fabric, with a specified type of phenolic resin, drying the impregnated web, and curing the phenolic resin under application of pressure and heat.
However, none of the prior art literature pay consideration to carbonization or graphitization. Further, even if these composites are carbonized or graphitized, not only the carbonization yield is low but also the heat conductivity, electrical conductivity and mechanical strength are insufficient, thus being inadequate for electrode use. Moreover, the carbonization or graphitization step must be followed by a machining step for the formation of said groove-shaped gas passageways.